DTMF is a well known convention for encoding control information on analog telephone lines that simultaneously carry voice signals. Despite the widespread usage of DTMF, there has been a continuing problem of accurately and consistently distinguishing between voice and DTMF. While a human listener can choose to ignore DTMF tones that coincide with a voice message, existing detection equipment occasionally accepts a voice signal as a DTMF signal, a phenomenon known as talk-off. This arises because the human voice includes components in the same spectrum as DTMF.
The problem of talk-off has other implications. For example, acoustically-coupled DTMF generators may cause errors in DTMF reception. Non-linearities in the acoustic coupling can distort DTMF and generate spurious signals which might be interpreted as voice. As another example, outgoing voice signals can mix with incoming DTMF signals in the transhybrid circuit. Because of the finite loss in the transhybrid circuit, the tones will contain additive voice noise, interfering with reception of the tones.
Various schemes have been proposed to address the three problems noted above. To counter talk-off, some have proposed increasing the minimum length of each DTMF signal. While this minimizes talk-off difficulties, it does not eliminate the problem. Moreover, some DTMF generators have fixed maximum lengths too short to be useable here. Further, an excessive length may cause user aggravation.
Some have also attempted to cure talk-off problems by requiting a minimal level of signal power outside of the DTMF band (below 680 Hz and above 2000 Hz), specifically at harmonic frequencies of DTMF tones. Although this yields some improvement, it does not completely eliminate talk-off. Also, measures to limit the acceptable amount of out-of-band power aggravates the finite transhybrid loss problem.
By narrowing the bandwidth of the individual DTMF receiver filters, a modest improvement in talk-off can be achieved. As with the two other schemes noted above, it does not completely eliminate talk-off. Also, there are limits as to how far one may carry this solution, as the Electronics Industry Association (EIA) DTMF standard requires that each filter have a minimum bandwidth of 1.5% of the center frequency to account for frequency variation in individual DTMF tones.
Several arrangements have been proposed to improve the performance of acoustically-coupled generators. Because the harmonics of DTMF generated by the microphone tend to have frequency components above the DTMF band, one solution suggests attenuating out-of-band components. This will result in an improvement although it is not fully reliable. Also, it is not compatible with the previously-noted scheme of requiring minimal out-of-band power to improve talk-off.
The performance of acoustically-coupled generators may also be improved by increasing the length of the tones, instructing the user to hold down each button for an extended period of time. The downside of this technique is that a long tone could be interpreted as two separate digits. This could occur, for instance, if there is a noise burst on the line, effectively "interrupting" the transmitted tone.
Finally, one can improve acoustic senders by literally banging the microphone end of the handset on a hard surface. This action serves to dislodge or loosen the carbon granules in the microphone's diaphragm that may have become packed due to moisture and migration on account of the action of the microphone. While this helps, many users are reluctant to do this.
The third area of focus is detection errors caused by the finite transhybrid loss. One way of minimizing the effect of this loss, described in U.S. Pat. No. 4,431,872, is to block outgoing signals upon detecting an incoming DTMF signal. This is sometimes referred to as early detect. While this statistically improves performance, recognition is not totally reliable. The outgoing signal may itself result in self-talk-off, distorting the outgoing signal.
U.S. Pat. No. 4,521,647 describes another solution to the problem of transhybrid loss. By modifying the outgoing signal whenever a short DTMF signal is received (early detect), the problem of distortion of the output from self-talk-off is greatly diminished. However, the results achieved are inferior to those of the method described in the previous paragraph.
An obvious way of minimizing the transhybrid loss problem is to decrease the transhybrid loss. One may choose to do this statically or dynamically, but either can be costly. Yet another method for minimizing the effects of finite transhybrid loss is attenuation out-of-band frequencies. This yields a modest improvement.
As noted, all of the methods described above result in some improvement, but do not fully solve the problem. Moreover, individual solutions to one of the three problems could aggravate one or both of the other problems.